Process for treating a natural gas containing carbon dioxide

ABSTRACT

The disclosure includes a process for treating a natural gas containing carbon dioxide wherein the natural gas is separated by a cryogenic process in order to provide, on the one hand, a stream of liquid carbon dioxide, containing hydrocarbons, and, on the other hand, purified natural gas; at least one part of the natural gas is cooled in a first heat exchanger and then in a second heat exchanger before the cryogenic process and/or before a reflux to the cryogenic process; at least one part of the stream of liquid carbon dioxide is recovered in order to provide a stream of recycled carbon dioxide; the stream of recycled carbon dioxide is divided into a first portion and a second portion; the first portion is expanded and then heated in the first heat exchanger, in order to provide a first stream of heated carbon dioxide; the second portion is cooled, then at least one part of the second portion is expanded and then heated in the second heat exchanger, in order to provide a second stream of heated carbon dioxide; at least some of the hydrocarbons contained in the first stream of heated carbon dioxide and in the second stream of heated carbon dioxide are recovered by liquid/gas separation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Entry of International ApplicationNo. PCT/IB2011/051879, filed on Apr. 28, 2011, which claims priority toFrench Patent Application Serial No. 1053340, filed on Apr. 29, 2010,both of which are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a cryogenic-type process for treatingnatural gas, with the aim of removing at least some of the carbondioxide that it contains, in which the hydrocarbons normally lost as aresult of the cryogenic treatment are largely recovered. The inventionalso relates to a plant suitable for implementing this process.

BACKGROUND

Within the context of producing natural gas or liquefied natural gas, itis necessary to purify said natural gas, originating from deposits, of acertain number of contaminants, primarily acidic gases such as hydrogensulphide (H₂S) and carbon dioxide (CO₂). In particular, carbon dioxidecan represent a major part of the gaseous mixture originating from adeposit of natural gas, up to more than 70% (in molar concentration).Several processes are known in the field for making it possible toreduce the carbon dioxide content of the natural gas.

The most usual treatment is based on the use of amine solvents. Thismethod makes possible a separation of the CO₂ that is very selectivevis-à-vis hydrocarbons; it makes it possible to lower the concentrationof CO₂ below the threshold of 50 ppm. But this method requires highenergy to regenerate the solvent. As a result, it is unsuitable if theoriginal gas has a high concentration of CO₂. Moreover, the regenerationis virtually atmospheric, and requires a compression that consumes a lotof energy if a reinjection of the separated CO₂ is envisaged (which isto be envisaged more and more routinely in view of the environmentalissues).

Another type of treatment is based on the use of semipermeablemembranes. The uses of these membranes for gases with an average CO₂content have developed significantly in the last few years. Membranetreatment is advantageous for significant concentrations of CO₂ and fora certain range of “feed-to-retentate” partial pressure ratios. However,when the CO₂ specifications are relatively low, the associated losses ofmethane can become considerable. It is also possible to provide severalstages of membranes for concentrating the CO₂ in the permeate, whichmakes it necessary to provide intermediate compressions of the permeate.The reinjection of the CO₂, if sought, requires an additionalcompression, from the low pressure of the final permeate, which furtherincreases the energy bill for this type of process.

Cryogenic processes constitute another type of treatment. The higher theconcentration of CO₂ in the original gas, the greater their advantage interms of energy. An example of a cryogenic process is shown in U.S. Pat.No. 4,152,129. However, due to the possible crystallization of the CO₂and/or the critical conditions at the head of the column, such a processdoes not allow stringent CO₂ requirements to be met. A finishingtreatment, for example of the amines type, is therefore essential if astrict CO₂ specification is required.

Certain variants of cryogenic treatment have been presented morerecently, in particular the process called “CFZ” (“Controlled FreezeZone”), the particular feature of which is to allow a crystallization ofthe CO₂ in the problematic zone of the column, which makes it possibleto envisage very high specifications with very low treatmenttemperatures (about −90° or even −110° C.). On this point, reference maybe made for example to U.S. Pat. No. 4,533,372.

Another variant of cryogenic treatment has been developed by Cool EnergyLimited. This process, called “CryoCell”, makes it possible, by means ofa cryogenic separation step, to meet specifications of about 2 to 3%CO₂, starting from a gas pretreated by cryogenic distillation, ordirectly for crude gases with an average concentration of CO₂ (typically25 to 35%). This process uses a liquefaction of the gas under pressure,then an expansion of the fluid which creates an intense cold and apartial crystallization of the CO₂. The liquid and solid fractions arerecovered in a flask designed for certain methods of application,keeping the bottom temperature in the liquid range. WO 2007/030888, WO2008/095258 and WO 2009/144275 illustrate this technique.

Another variant of cryogenic treatment is constituted by the family ofso-called “Ryan Holmes” processes. These processes, which make possiblea fairly complete recovery of the C3+ hydrocarbons, use 3 or 4distillation columns, depending on the nature of the gas, and as aresult prove to be relatively complex and costly in terms of investmentand consumption.

A drawback of these cryogenic methods is that they separate thecomponents according to their volatility and therefore, with the liquidCO₂, trap virtually all of the C3+ hydrocarbons contained in the naturalgas. This constitutes a sometimes very great handicap depending on thecomposition of the gas. It is estimated that 8 to 15% by mass of thehydrocarbons are generally lost when a separation of the CO₂ bydistillation is implemented; furthermore, the majority of thehydrocarbons lost are hydrocarbons with an intermediate molar mass,therefore the most valuable.

WO 99/01707 relates to a variant of the process called “CFZ”, in whichsome of the stream of liquid CO₂ recovered at the foot of thedistillation column is expanded, then used to cool the natural gasbefore it enters the distillation column in two successive heatexchangers. Between the two heat exchangers, the stream of CO₂ undergoesa gas/liquid separation, only the liquid portion being expanded thenguided to the second heat exchanger (the gaseous portion beingcompressed before finally being removed). At the outlet of the secondheat exchanger, another gas/liquid separation is provided: the gaseousphase is compressed before finally being removed, while the liquid phaseprovides a recovery of the condensates trapped in the stream of CO₂.

This technique makes it possible to limit the hydrocarbon losses in thestream of liquid CO₂ and could be applied to any process forcryogenically separating the CO₂ which traps C3+ hydrocarbons in theliquid CO₂. On the other hand, a drawback of the technique proposed inthis document is that the composition of the stream (mostly CO₂) in thesuccessive heat exchangers varies, the stream becoming progressivelyricher in heavy fractions. This leads to an increased risk ofcrystallization, in particular of the paraffinic hydrocarbons, andparticularly in the last heat exchanger in the cold cycle, thetemperature of which is the lowest. This is why the document providesthe alternative of a rectification column for the natural gas at theinlet of the plant in order to avoid these problems, so as to removesome of the heavy compounds upstream. This method is extremely complexand difficult to implement, since it requires an additionalfractionation of all of the gas.

There is therefore a real need to develop a treatment that makes itpossible to effectively reduce the hydrocarbon losses for these types ofcryogenic separation of CO₂, in a manner that is simple to implement.

SUMMARY

In the first place the invention relates to a process for treating anatural gas containing carbon dioxide in which:

-   -   the natural gas is separated by a cryogenic process in order to        provide, on the one hand, a stream of liquid carbon dioxide        containing hydrocarbons and, on the other hand, purified natural        gas;    -   at least one part of the natural gas is cooled in a first heat        exchanger then in a second heat exchanger before said cryogenic        process and/or before a reflux to said cryogenic process;    -   at least one part of the stream of liquid carbon dioxide is        recovered in order to provide a stream of recycled carbon        dioxide;    -   the stream of recycled carbon dioxide is divided into a first        portion and a second portion;    -   the first portion is expanded then is heated in the first heat        exchanger, in order to provide a first stream of heated carbon        dioxide;    -   the second portion is cooled, then at least one part of the        second portion is expanded then is heated in the second heat        exchanger, in order to provide a second stream of heated carbon        dioxide;    -   at least some of the hydrocarbons contained in the first stream        of heated carbon dioxide and in the second stream of heated        carbon dioxide are recovered by liquid/gas separation.

According to an embodiment:

-   -   at least one part of the natural gas is cooled in a third heat        exchanger before the cryogenic process and/or before a reflux to        the cryogenic process;    -   the second portion of the stream of recycled carbon dioxide is        divided into a third portion and a fourth portion;    -   the third portion is expanded then is heated in the second heat        exchanger, in order to provide the second stream of heated        carbon dioxide;    -   the fourth portion is cooled then expanded, then it is heated in        the third heat exchanger, in order to provide a third stream of        heated carbon dioxide;    -   at least some of the hydrocarbons contained in the third stream        of heated carbon dioxide are recovered by liquid/gas separation.        According to an embodiment, the first heat exchanger, the second        heat exchanger and, if applicable, the third heat exchanger        operate at different temperatures, and preferably the first heat        exchanger operates at a higher temperature than the second heat        exchanger and, if applicable, the second heat exchanger operates        at a higher temperature than the third heat exchanger. According        to an embodiment, said cryogenic process is a distillation.

According to an embodiment:

-   -   the cooling of the second portion of the stream of recycled        carbon dioxide is carried out in the second heat exchanger;    -   the cooling of the fourth portion of the stream of recycled        carbon dioxide, if applicable, is carried out in the third heat        exchanger; and    -   preferably the stream of recycled carbon dioxide is cooled in        the first heat exchanger before being divided into the first        portion and the second portion.        According to an embodiment, the purified natural gas is heated,        if applicable first in the third heat exchanger, then in the        second heat exchanger, then in the first heat exchanger.

According to an embodiment:

-   -   the first stream of heated carbon dioxide undergoes a liquid/gas        separation in a first separation flask in order to provide a        first gaseous phase and a first liquid phase;    -   the first liquid phase is expanded;    -   the second stream of heated carbon dioxide and the first        expanded liquid phase undergo a liquid/gas separation in a        second separation flask in order to provide a second gaseous        phase and a second liquid phase; and, preferably:        -   the second liquid phase is expanded;        -   the third stream of heated carbon dioxide and the second            expanded liquid phase undergo a liquid/gas separation in a            third separation flask in order to provide a third gaseous            phase and a third liquid phase.            According to an embodiment, the second liquid phase or, if            applicable, the third liquid phase, undergoes a step of            stabilizing the condensates in order to provide a liquid            phase rich in hydrocarbons and a gaseous phase rich in            carbon dioxide, said gaseous phase rich in carbon dioxide            preferably undergoing a liquid/gas separation in the second            separation flask or, if applicable, in the third separation            flask. According to an embodiment, the first gaseous phase,            the second gaseous phase and, if applicable, the third            gaseous phase are compressed and cooled in order to provide            an outlet stream of carbon dioxide, which is optionally            mixed with at least one part of the stream of liquid carbon            dioxide.

According to an embodiment:

-   -   one part of the second liquid phase is mixed with the second        portion of the stream of recycled carbon dioxide or, if        applicable, one part of the third liquid phase is mixed with the        fourth portion of the stream of recycled carbon dioxide; and/or    -   one part of the outlet stream of carbon dioxide is mixed with        the stream of recycled carbon dioxide.

Another subject of the invention is a plant for treating natural gascontaining carbon dioxide comprising:

-   -   a cryogenic separation unit;    -   at least one line for natural gas connected at the inlet of the        cryogenic separation unit;    -   a line for liquid carbon dioxide and a line for purified natural        gas originating from the cryogenic separation unit;    -   a first heat exchanger passed through by at least one of the        lines for natural gas connected at the inlet of the cryogenic        separation unit;    -   a second heat exchanger passed through by at least one of the        lines for natural gas connected at the inlet of the cryogenic        separation unit or by a line for natural gas connected at the        outlet of the cryogenic separation unit and feeding a reflux        system;    -   a line for recycled carbon dioxide originating from the line for        liquid carbon dioxide;    -   a line for the first portion and a line for the second portion        originating from the line for recycled carbon dioxide,        -   the line for the first portion being equipped with expansion            means and then passing through the first heat exchanger;        -   the line for the second portion being equipped with cooling            means;    -   a line for the third portion originating from the line for the        second portion, said line for the third portion being equipped        with expansion means and then passing through the second heat        exchanger;    -   gas/liquid separation means fed by the line for the first        portion and the line for the third portion.

According to an embodiment:

-   -   the plant comprises a third heat exchanger passed through by at        least one of the lines for natural gas connected at the inlet of        the cryogenic separation unit or by a line for natural gas        connected at the outlet of the cryogenic separation unit and        feeding a reflux system;    -   the line for the second portion divides into the line for the        third portion and a line for the fourth portion;    -   the line for the fourth portion is equipped with cooling means,        expansion means, and then passes through the third heat        exchanger; and    -   the plant comprises gas/liquid separation means fed by the line        for the fourth portion.

According to an embodiment:

-   -   the cooling means on the line for the second portion are        constituted by the second heat exchanger;    -   if applicable, the cooling means on the line for the fourth        portion are constituted by the third heat exchanger; and    -   preferably the line for recycled carbon dioxide is equipped with        cooling means constituted by the first heat exchanger, before        dividing into the line for the first portion and the line for        the second portion.        According to an embodiment, the cryogenic separation unit is a        distillation unit. According to an embodiment, the line for        purified natural gas passes, if applicable, through the third        heat exchanger, then the second heat exchanger, then the first        heat exchanger.

According to an embodiment:

-   -   the gas/liquid separation means comprise a first separation        flask and a second separation flask;    -   the first separation flask is fed by the line for the first        portion;    -   a line for the first gaseous phase and a line for the first        liquid phase are connected at the outlet of the first separation        flask;    -   the line for the first liquid phase is equipped with expansion        means;    -   the second separation flask is fed by the line for the third        portion and by the line for the first liquid phase;    -   a line for the second gaseous phase and a line for the second        liquid phase are connected at the outlet of the second        separation flask; and preferably:        -   the line for the second liquid phase is equipped with            expansion means;        -   the line for the fourth portion and the line for the second            liquid phase feed a third separation flask;        -   a line for the third gaseous phase and a line for the third            liquid phase are connected at the outlet of the third            separation flask.

According to an embodiment, the line for the second liquid phase or, ifapplicable, the line for the third liquid phase, feeds a condensatestabilization unit, at the outlet of which a line for liquid phase richin hydrocarbons and a line for gaseous phase rich in carbon dioxide areconnected, said line for gaseous phase rich in carbon dioxide preferablyfeeding the second separation flask or, if applicable, the thirdseparation flask. According to an embodiment, the line for the firstgaseous phase, the line for the second gaseous phase and, if applicable,the line for the third gaseous phase feed compression means and join inan outlet line for carbon dioxide, said outlet line for carbon dioxidepreferably being equipped with cooling means and preferably joining aline for non-recycled carbon dioxide originating from the line forliquid carbon dioxide, in order to form a line for final carbon dioxide.

According to an embodiment, the plant comprises:

-   -   an additional line for hydrocarbons equipped with pumping means,        connected at the outlet of the second separation flask and        returning to the line for the second portion upstream of the        second heat exchanger or, if applicable, connected at the outlet        of the third separation flask and returning to the line for the        fourth portion upstream of the third heat exchanger; and/or    -   an additional line for carbon dioxide equipped with a valve,        reaching from the outlet line for carbon dioxide to the line for        recycled carbon dioxide.        According to an embodiment, the process as described above is        implemented in the above-mentioned plant.

The present invention makes it possible to overcome the drawbacks of thestate of the art. More particularly it provides a treatment for naturalgas whereby the carbon dioxide content can be significantly reduced.Said treatment is implemented while limiting the losses of hydrocarbons,in particular the C3+ compounds trapped with the stream of liquid carbondioxide.

This is achieved, on the one hand, by recycling at least some of thecarbon dioxide originating from a distillation (or more generally from acryogenic process) and by using this carbon dioxide rich in C3+ as arefrigerant in an open refrigeration cycle in order to produce thefrigories necessary for the cryogenic process, i.e. by making a heatexchange necessary (in several steps) between the carbon dioxide used inthe open refrigeration cycle and the natural gas; on the other hand, byrecovering the hydrocarbons trapped in the carbon dioxide from the openrefrigeration cycle by a simple gas/liquid separation after the heatexchange with the natural gas, the composition of the stream of carbondioxide from the open refrigeration cycle remaining constant during thedifferent steps of said heat exchange.

According to certain particular embodiments, the invention also has oneor preferably several of the advantageous characteristics listed below.

-   -   The invention does not require major new equipment compared with        a plant equipped with a standard, closed-loop, cooling unit,        optionally with the exception of the equipment for stabilizing        the condensates.    -   The invention makes it possible to recover the CO₂ in liquid        form at the end of the refrigeration cycle; it can then be        pressurized by simple pumping for injection into geological        structures (unlike the processes based on an amine solvent or on        a semipermeable membrane).    -   The process of the invention is particularly useful and        appropriate for a natural gas comprising an average or high CO₂        content and comprising a significant fraction of C3+        hydrocarbons.    -   The invention is particularly suitable for offshore        applications, where the use of C2/C3 refrigerant, which is        highly flammable, is not desirable for safety reasons.    -   The renewable nature of the refrigerant used according to the        invention makes it possible to work with a minimum buffer stock,        without fearing the consequences of multiple decompressions of        the cycle. Thus the invention makes it possible to eliminate        logistical problems with regard to the refrigerant.    -   The invention can make it possible to recover a significant        fraction of heavy hydrocarbons (C3+). Thus, in the example        provided below, the invention makes it possible to increase the        production of hydrocarbons, in the form of highly valuable        stabilized condensates, by approximately 3% by mass.    -   Compared with the process described in WO 99/01707, the        invention has the advantage of limiting the risks of        crystallization in the refrigeration cycle, linked to the        concentration of heavy paraffinic hydrocarbons, and therefore of        avoiding, in the vast majority of cases, the need for a        fractionation of the natural gas upstream of the cryogenic        process.

DRAWINGS

FIG. 1 diagrammatically shows an embodiment of a plant according to theinvention.

DETAILED DESCRIPTION

The invention will now be described in greater detail and in anon-limitative fashion in the following description. All pressures aregiven in absolute values. All percentages are given as molar values,unless otherwise indicated. The terms “upstream” and “downstream” referto the direction of flow of the fluids in the plant.

Plant

With reference to FIG. 1, the plant according to the invention comprisesa feed line for natural gas 1. This feed line for natural gas 1preferably passes through a pretreatment unit 57, which can includepre-cooling means and/or dehydration means and/or gas/liquid separationmeans and/or fractionation means. It is preferred, for reasons ofsimplicity, that the plant be without fractionation means anddeacidification means in the pretreatment unit 57.

The feed line for natural gas 1 feeds (indirectly) a cryogenicseparation unit 35. By “cryogenic separation unit” is meant a set ofmeans capable of separating carbon dioxide from methane with a supply ofcold at an operating temperature below or equal to −40° C.

Preferably, the cryogenic separation unit 35 is a distillation unit and,more precisely, in the embodiment shown, it is a standard distillationcolumn equipped with a reboiler 32 at the foot. Heat exchange meansbetween the feed line for natural gas 1 and the reboiler 32 areprovided; the feed line for natural gas 1 opens into a gas/liquidseparator 31. Two lines for natural gas 33, 34, namely a line forgaseous fraction 33 and a line for liquid fraction 34, are connected atthe outlet of the gas/liquid separator 31.

The line for gaseous fraction 33 and the line for liquid fraction 34respectively open into the cryogenic separation unit 35, at differentstages. Each of these two lines is equipped with expansion means;moreover, the line for gaseous fraction 33 passes successively through afirst heat exchanger 36 and a second heat exchanger 37 before passingthrough the above-mentioned expansion means and opening into thecryogenic separation unit 35.

A line for liquid carbon dioxide 10 is connected at the foot of thecryogenic separation unit 35, and a line for natural gas 39, feeding areflux system, is connected at the head of the cryogenic separation unit35. More precisely, the line for natural gas 39 passes through a thirdheat exchanger 38 then feeds a gas/liquid separator 40. At the outlet ofthis gas/liquid separator 40, there are connected, at the foot on theone hand, a reflux line 41 equipped with pumping means and returning tothe cryogenic separation unit 35 and, at the head on the other hand, aline for purified natural gas 99.

The line for purified natural gas 99 passes successively through thethird heat exchanger 38, the second heat exchanger 37 and the first heatexchanger 36. On the diagram, the streams passing through the heatexchangers from left to right give off heat and the streams passingthrough the heat exchangers from right to left absorb heat. Thus, thecooling of the heat exchangers 36, 37, 38 is ensured by the line forpurified natural gas 99 and by the open refrigeration cycle describedbelow and containing a stream rich in carbon dioxide. The line forpurified natural gas 99 can be followed by recompression means.

If necessary, additional treatment means (and in particular additionaldeacidification means) can be provided from the line for purifiednatural gas 99, if a finishing purification of the gas is necessary.Such additional treatment means (generally situated downstream offractionation means) can comprise means for treating the carbon dioxideaccording to any one of the techniques known in the state of the art(for example scrubbing with amine solvent, separation by membrane,etc.). This can prove useful in the case of a gas comprising a very highCO₂ content.

Downstream, this line for purified natural gas 99 can be linked to thegas transport and/or distribution network, or feed a natural gasliquefaction unit. Moreover, the line for liquid carbon dioxide 10divides into two branches, namely a line for non-recycled carbon dioxide11 and a line for recycled carbon dioxide 12. The line for recycledcarbon dioxide 12 passes through the first heat exchanger 36. Then itdivides into two branches, namely a line for the first portion 13 and aline for the second portion 42. The line for the second portion 42passes through the second heat exchanger 37 then itself divides into twobranches, namely a line for the third portion 16 and a line for thefourth portion 19. The line for the fourth portion 19 passes through thethird heat exchanger 38 a first time.

Expansion means 43 are provided on the line for the first portion 13,which then passes through the first heat exchanger 36, before feeding afirst separation flask 47. Similarly, expansion means 45 are provided onthe line for the third portion 16, which then passes through the secondheat exchanger 37, before feeding a second separation flask 48. Finally,the line for the fourth portion 19 passes through the third heatexchanger 38 a second time, expansion means 46 being provided on theline for the fourth portion 19 between its two passages through the heatexchanger 38; finally, the line for the fourth portion 19 feeds a thirdseparation flask 49.

The three separation flasks 47, 48, 49 are suitable for carrying out aliquid/gas separation and they are connected in cascade. In other words,at the outlet of the first separation flask 47 there are connected aline for the first gaseous phase 15 (at the head) and a line for thefirst liquid phase 14 (at the foot), said line for the first liquidphase 14 feeding the second separation flask 48 after having passedthrough expansion means 58; similarly, at the outlet of the secondseparation flask 48 there are connected a line for the second gaseousphase 18 (at the head) and a line for the second liquid phase 17 (at thefoot), said line for the second liquid phase 17 feeding the thirdseparation flask 49 after passing through expansion means 59. At theoutlet of the third separation flask 49 there are connected a line forthe third gaseous phase 23 (at the head) and a line for the third liquidphase 20 (at the foot).

The line for the third liquid phase 20 is equipped with pumping meansand feeds a condensate stabilization unit 55. This condensatestabilization unit 55 can be a distillation column or, preferably, adistillation half-column, i.e. a column equipped with a reboiler 56 atthe foot, but without a cooling and reflux system at the head.

At the outlet of the condensate stabilization unit 55 there areconnected, on the one hand, a line for liquid phase rich in hydrocarbons21 at the foot and a line for gaseous phase rich in carbon dioxide 22 atthe head. The line for gaseous phase rich in carbon dioxide 22 returnsto the third separation flask 49. The line for liquid phase rich inhydrocarbons 21 can open into treatment means (for example fractionationmeans) and/or means for storing condensates.

The line for the third gaseous phase 23 feeds a first compressor 50, atthe outlet of which a first intermediate line 24 is connected. Thisfirst intermediate line 24 is joined by the line for the second gaseousphase 18, at the inlet of a second compressor 51. A second intermediateline 25 is connected at the outlet of the second compressor 51. Thissecond intermediate line 25 is joined by the line for the first gaseousphase 15, at the inlet of a third compressor 52. An outlet line forcarbon dioxide 26 is connected at the outlet of the third compressor 52.

The outlet line for carbon dioxide 26 is equipped with cooling means 53and joins the line for non-recycled carbon dioxide 11 in order to form aline for final carbon dioxide 27. Pumping means can be provided on this.The line for final carbon dioxide 27 can open into downstream treatmentmeans, for example means for injection into an underground formation.

Process

The natural gas which is treated by the process according to theinvention is a gaseous mixture (which may contain a minority liquidfraction) comprising at least methane and CO₂. Preferably, this gaseousmixture comprises at least 5% methane, and generally at least 10% or atleast 15% or at least 20% methane or at least 25% methane (molarproportions relative to the natural gas). Preferably, this gaseousmixture comprises at least 10% CO₂, and generally at least 20% CO₂ or atleast 30% CO₂ or at least 40% CO₂ or at least 50% CO₂ or at least 60%CO₂ or at least 70% CO₂ (molar proportions relative to the natural gas).The natural gas also contains C3+ hydrocarbons (comprising at least 3carbon atoms), preferably in a proportion by mass greater than or equalto 1% or 2% or 3% or 4% or 5% relative to the methane.

The natural gas optionally undergoes one or more preliminary treatments(in the pretreatment unit 57) with the aim of removing its solidcontaminants or its liquid fraction, dehydrating it and/or pre-coolingit and/or reducing its hydrogen sulphide content. According to apreferred embodiment, the natural gas does not undergo any treatmentwith the specific aim of reducing its CO₂ content prior to the cryogenicseparation. In the embodiment shown, the natural gas is first cooled byheat exchange in the reboiler 32 of the cryogenic separation unit 35,then it undergoes a separation into a gaseous phase and a liquid phasein the gas/liquid separator 31. These two phases are introduced atdifferent stages of the cryogenic separation unit 35, after anexpansion.

A stream of liquid carbon dioxide is recovered at the foot of thecryogenic separation unit 35 in the line for liquid carbon dioxide 10.By “stream of carbon dioxide” is meant, within the context of thepresent description, a mixture comprising mostly CO₂ and comprising aminority proportion of other compounds, in particular C3+ hydrocarbons.

The cooling needed to implement the cryogenic separation is ensured bythe multi-stage open refrigeration cycle (at least two heat exchangers)which is fed by at least one part of the liquid carbon dioxide (streamof recycled carbon dioxide). In the embodiment shown, the refrigerationis carried out in the three heat exchangers 36, 37, 38 operating atdecreasing temperatures, the heat exchangers 36 and 37 typicallyfunctioning at between −40° C. and 0° C., and the heat exchanger 38typically functioning at between −60° C. and −45° C. (temperature of therefrigeration fluid after expansion). More precisely, the gaseous phaseof the natural gas is cooled in the first heat exchanger 36 and thesecond heat exchanger 37.

The third heat exchanger 38 serves to cool the reflux of the cryogenicseparation, i.e. to cool the stream of natural gas leaving the cryogenicseparation unit 35 at the head. After this cooling, the stream ofnatural gas undergoes a separation in the gas/liquid separator 40producing a stream of liquid phase which is pumped and returned to thecryogenic separation (reflux line 41), and a stream of purified naturalgas which is recovered in the line for purified natural gas 99. In theembodiment shown, the stream of purified natural gas is heated in thethree heat exchangers 38, 37, 36 successively, which makes it possibleto recover the frigories available therein.

With regard to the functioning of the refrigeration cycle, the stream ofrecycled carbon dioxide undergoes a first cooling in the first heatexchanger 36, then it is divided into two liquid streams, namely a firstportion and a second portion. The first portion is cooled by expansion,and it then returns to the first heat exchanger 36, in which it absorbsheat originating from the natural gas upstream of the cryogenicseparation (and also heat originating from the stream of recycled carbondioxide before expansion). The second portion undergoes a second coolingin the second heat exchanger 37, then it is divided into two liquidstreams, namely a third portion and a fourth portion. The third portionis cooled by expansion, and it then returns to the second heat exchanger37, in which it absorbs heat originating from the natural gas upstreamof the cryogenic separation (and also heat originating from the streamof recycled carbon dioxide before expansion). The fourth portionundergoes a third cooling in the third heat exchanger 38, then it iscooled by expansion, and it then returns to the third heat exchanger 38,in which it absorbs heat originating from the natural gas at the levelof the reflux of the cryogenic separation (and also heat originatingfrom the stream of recycled carbon dioxide before expansion).

A first, second and third stream of heated carbon dioxide are thereforerecovered at the outlet of the first, second and third heat exchanger36, 37, 38 respectively. A significant part of the C3+ hydrocarbonscontained in these streams is recovered by liquid/gas separation carriedout on these streams. The liquid/gas separation is carried out by meansof the first, second and third separation flasks 47, 48, 49, operatingat decreasing pressures. The typical operating pressures are 10 bar to40 bar for the separation flasks 47 and 48, and bar to 10 bar for theseparation flask 49.

Each separation flask (respectively the first, second or third) producesa liquid phase (respectively the first, second or third) and a gaseousphase (respectively the first, second or third). The heavy hydrocarbons(essentially C4+) are mostly in the liquid phase. The first liquid phaseis expanded and sent to the second separation flask 48 operating at alower pressure than the first, and similarly the second liquid phase isexpanded and sent to the third separation flask 49 operating at a lowerpressure than the second. Thus, the heavy hydrocarbons trapped in thestream of CO₂ tend to concentrate in the bottom of the third separationflask 49 functioning at the lowest pressure, where they can easily berecovered in the third liquid phase.

An additional purification step (stabilization of the condensates) canbe implemented, as shown, by means of the condensate stabilizationcolumn 55. A liquid phase rich in hydrocarbons is recovered at the footthereof and a gaseous phase rich in carbon dioxide, which is returned tothe separation flask at the lowest pressure, is recovered at the head.Each gaseous phase originating from the different separation flasks,depleted of heavy hydrocarbons, is compressed; these different gaseousphases are mixed, then the mixture is cooled and advantageously combinedwith the part of the liquid CO₂ that is not recycled for refrigeration.The stream of final liquid CO₂ can be pumped and injected into anunderground formation, or else be used or otherwise turned to account.

Variants

The plant according to the invention and the process according to theinvention can be varied from the embodiment described above in severalways. For example, it is possible to provide an additional line forcarbon dioxide 54 equipped with a valve reaching from the outlet linefor carbon dioxide 26 (typically downstream of the cooling means 53) tothe line for recycled carbon dioxide 12. This characteristic makes itpossible to compensate for any lack of refrigerant in the multi-stagerefrigeration system, making it possible to recycle part of the CO₂stream used for the refrigeration.

It is also possible to provide an additional line for hydrocarbons 44(optionally equipped with a valve) connected at the outlet of the thirdseparation flask 49 at the foot, equipped with pumping means andreturning to the line for the fourth portion 19, upstream of the firstpassage into the third heat exchanger 38. Thus, part of the third liquidphase can be recycled in the CO₂ stream used for the refrigeration. Thischaracteristic makes it possible to avoid any risk of crystallization atthe coldest point, while enriching the expanded stream passing throughthe third heat exchanger 38 with hydrocarbons.

Moreover, the above description was made in relation to an openrefrigeration cycle with three stages. This is the variant that makes anoptimum functioning of the system possible. However, it is also possibleto provide a cycle with two stages or, alternatively, with four or morestages.

In the case of a system with two stages, compared with the abovedescription: the third heat exchanger 38 and the third separation flask49 are omitted, as are the associated components, namely the line forthe fourth portion 19, the line for the third gaseous phase 23, thefirst compressor 50 and the first intermediate line 24. The line for thesecond liquid phase 17 then merges with the line for the third liquidphase 20 and therefore directly feeds the condensate stabilization unit55.

In the case of a system with four or more stages, compared with theabove description, at least one additional heat exchanger (suitable forcooling the natural gas upstream of the cryogenic separation unit or inthe reflux of the latter) and at least one additional separation flaskare added; at least one additional division of the line originating fromthe line for recycled carbon dioxide 12, equipped with expansion meansand feeding the additional separation flask are also added; and, at theoutlet of the (or of each) additional separation flask, there areprovided an additional line for gaseous phase, associated with anadditional compressor, and an additional line for liquid phase, equippedwith expansion means and feeding the following separation flask (i.e.operating at lower pressure).

Moreover, in the embodiment shown, the line for natural gas 33 passinginto the first heat exchanger 36 and the second heat exchanger 37originates from the gas/liquid separator 31 and feeds the cryogenicseparation unit 35; and the natural gas line 39 passing into the thirdheat exchanger 38 forms part of the reflux system of the cryogenicseparation unit 35, since it originates from the head of the cryogenicseparation unit 35 and feeds the gas/liquid separator 40 to which thereflux line 41 is connected at the foot. However, this distribution canbe modified according to, on the one hand, the number of heat exchangersand, on the other hand, the operating parameters of the plant.

For example, the line for natural gas 33 originating from the gas/liquidseparator 31 and feeding the cryogenic separation unit 35 can passthrough a single heat exchanger (in particular if the refrigerationcycle comprises only two heat exchangers, in which case the second heatexchanger can be associated with the reflux system of the cryogenicseparation unit 35). Conversely, this line for natural gas 33 can passthrough more than two heat exchangers. Another variant is for all of theheat exchangers to be associated with the line for natural gas 33originating from the gas/liquid separator 31 and feeding the cryogenicseparation unit 35, in which case the reflux system of the cryogenicseparation unit 35 is equipped with additional cooling means (replacingthe third heat exchanger described above).

The cryogenic separation unit 35 can be a standard distillation column,suitable for the cryogenic separation of CO₂, as described above. But itcan also be a distillation column suitable for functioning undersolids-forming conditions (“CFZ”-type column, such as described forexample in U.S. Pat. No. 4,533,372 or WO 99/01707). The cryogenicseparation unit 35 can also comprise liquefaction means suitable forliquefying the gas under pressure, means for expanding the fluidsuitable for creating an intense cold and a partial crystallization ofthe CO₂, and means for recovering a liquid fraction and a solid fractioncomprising a flask suitable for maintaining a bottom temperature in theliquid range (“cryocell”-type distillation unit as described for examplein WO 2007/030888, WO 2008/095258 and WO 2009/144275). In this case, itis advantageous to provide a stabilization column on the line for liquidcarbon dioxide 10, suitable for recovering the light hydrocarbons (inparticular methane) present in the liquid CO₂.

EXAMPLE

The following example illustrates the invention without limiting it. Anumerical simulation was carried out in order to characterize thefunctioning of a plant corresponding to FIG. 1. Tables 1a, 1b, 1c, 1d,2a, 2b, 2c and 2d below give the composition of the starting natural gasas well as the flow rates obtained and the composition of the streamobtained in different lines of the plant. The conditions in lines 13,16, 19 were recorded at the outlet of the respective heat exchangers 36,37, 38. The conditions in lines 14, 17, 20 were recorded at the outletof the respective separation flasks 47, 48, 49 and before expansion orpumping. The conditions in line 10 were recorded before pumping.

TABLE 1a general data and molar data Line of the plant 1 99 10 11 12Liquid (L) or G + L G + L L L L gaseous (G) state Temperature 4.7419.948 9.948 (° C.) Pressure 40.680 80.000 80.000 (bar) Molecular 35.48521.904 43.878 43.878 43.878 weight Flow rate 35260.954 12067.20223168.041 35.100 23132.941 (kmol/h) Composition (mole %) N₂ 0.50 1.460.00 0.00 0.00 CO₂ 71.00 20.00 97.53 97.53 97.53 H₂S 0.50 0.10 0.71 0.710.71 Methane 27.00 77.93 0.50 0.50 0.50 Ethane 0.60 0.49 0.66 0.66 0.66Propane 0.20 0.02 0.29 0.29 0.29 Heptane 0.20 0.00 0.30 0.30 0.30

TABLE 1b general data and molar data (continued) Line of the plant 13 1415 16 17 Liquid (L) or G + L L G G + L L gaseous (G) state Temperature6.891 6.637 6.637 −11.001 −11.425 (° C.) Pressure 27.626 27.426 27.42613.723 13.520 (bar) Molecular 43.878 66.018 43.790 43.878 75.424 weightFlow rate 12435.033 49.604 12385.430 8522.613 70.771 (kmol/h)Composition (mole %) N₂ 0.00 0.00 0.00 0.00 0.00 CO₂ 97.53 57.83 97.6997.53 41.85 H₂S 0.71 0.77 0.71 0.71 0.61 Methane 0.50 0.08 0.50 0.500.05 Ethane 0.66 0.51 0.66 0.66 0.31 Propane 0.29 1.34 0.29 0.29 1.08Heptane 0.30 39.47 0.15 0.30 56.11

TABLE 1c general data and molar data (continued) Line of the plant 18 1920 21 22 Liquid (L) or G G + L L L G gaseous (G) state Temperature−11.425 −33.026 −32.595 168.547 −30.301 (° C.) Pressure 13.520 5.6775.477 6.000 6.000 (bar) Molecular 43.745 43.878 82.729 99.655 43.780weight Flow rate 8501.446 2175.294 66.255 46.184 20.071 (kmol/h)Composition (mole %) N₂ 0.00 0.00 0.00 0.00 0.00 CO₂ 97.77 97.53 29.500.00 97.38 H₂S 0.71 0.71 0.46 0.00 1.51 Methane 0.50 0.50 0.02 0.00 0.08Ethane 0.66 0.66 0.16 0.00 0.53 Propane 0.29 0.29 0.82 0.98 0.47 Heptane0.07 0.30 69.03 99.02 0.03

TABLE 1d general data and molar data (continued) Line of the plant 23 2425 26 27 Liquid (L) or G G G L L gaseous (G) state Temperature −32.59540.028 61.729 33.000 32.988 (° C.) Pressure 5.477 13.520 27.926 80.00080.000 (bar) Molecular 43.722 43.722 43.740 43.767 43.767 weight Flowrate 2199.881 2199.881 10701.327 23086.758 23121.857 (kmol/h)Composition (mole %) N₂ 0.00 0.00 0.00 0.00 0.00 CO₂ 97.79 97.79 97.7797.73 97.73 H₂S 0.72 0.72 0.71 0.71 0.71 Methane 0.50 0.50 0.50 0.500.50 Ethane 0.66 0.66 0.66 0.66 0.66 Propane 0.30 0.30 0.29 0.29 0.29Heptane 0.03 0.03 0.06 0.11 0.11

TABLE 2a data by mass Line of the plant 1 99 10 11 12 Flow rate (kg/h)1282025.1 264320.9 1016579.3 1540.1 1015039.2 Composition (% by mass) N₂0.39 1.87 0.00 0.00 0.00 CO₂ 85.94 40.18 97.83 97.83 97.83 H₂S 0.47 0.160.55 0.55 0.55 Methane 11.91 57.08 0.18 0.18 0.18 Ethane 0.50 0.67 0.450.45 0.45 Propane 0.24 0.05 0.29 0.29 0.29 Heptane 0.55 0.00 0.70 0.700.70

TABLE 2b data by mass (continued) Line of the plant 13 14 15 16 17 Flowrate (kg/h) 545630.8 3274.8 542356.1 373959.6 5337.8 Composition (% bymass) N₂ 0.00 0.00 0.00 0.00 0.00 CO₂ 97.83 38.55 98.18 97.83 24.42 H₂S0.55 0.40 0.55 0.55 0.27 Methane 0.18 0.02 0.18 0.18 0.01 Ethane 0.450.23 0.45 0.45 0.12 Propane 0.29 0.89 0.29 0.29 0.63 Heptane 0.70 59.900.34 0.70 74.54

TABLE 2c data by mass (continued) Line of the plant 18 19 20 21 22 Flowrate (kg/h) 371896.6 95448.7 5481.2 4602.5 878.7 Composition (% by mass)N₂ 0.00 0.00 0.00 0.00 0.00 CO₂ 98.36 97.83 15.69 0.00 97.89 H₂S 0.550.55 0.19 0.00 1.17 Methane 0.18 0.18 0.00 0.00 0.03 Ethane 0.46 0.450.06 0.00 0.36 Propane 0.29 0.29 0.44 0.43 0.47 Heptane 0.16 0.70 83.6299.57 0.07

TABLE 2d data by mass (continued) Line of the plant 23 24 25 26 27 Flowrate (kg/h) 96184.0 96184.0 468080.6 1010436.7 1011976.8 Composition (%by mass) N₂ 0.00 0.00 0.00 0.00 0.00 CO₂ 98.43 98.43 98.37 98.27 98.27H₂S 0.56 0.56 0.55 0.55 0.55 Methane 0.18 0.18 0.18 0.18 0.18 Ethane0.46 0.46 0.46 0.45 0.45 Propane 0.31 0.31 0.30 0.29 0.29 Heptane 0.060.06 0.14 0.24 0.25

It is noted in this example that only 10% of the C7 hydrocarbons(representing the heavy paraffins) present in the liquid CO₂ passthrough the coldest heat exchanger. This illustrates the impact of theprocess, compared with the state of the art, where a cascaderefrigeration cycle would collect all of the heavy paraffins in thecoldest heat exchanger.

The invention claimed is:
 1. A process for treating a natural gascontaining carbon dioxide, the process comprising: separating thenatural gas by a cryogenic process in order to provide a stream ofliquid carbon dioxide containing hydrocarbons and purified natural gas;cooling at least one part of the natural gas: (a) in a first heatexchanger, (b) then in a second heat exchanger before the cryogenicprocess or before a reflux to the cryogenic process; recovering at leastone part of the stream of liquid carbon dioxide in order to provide astream of recycled carbon dioxide; dividing the stream of recycledcarbon dioxide into a first portion and a second portion; expanding andthen heating the first portion in the first heat exchanger, in order toprovide a first stream of heated carbon dioxide; cooling the secondportion, then expanding and then heating at least one part of the secondportion in the second heat exchanger, in order to provide a secondstream of heated carbon dioxide; and recovering at least some of thehydrocarbons contained in the first stream of heated carbon dioxide andin the second stream of heated carbon dioxide by liquid/gas separation;the first stream of heated carbon dioxide undergoing a liquid/gasseparation in a first separation flask in order to provide a firstgaseous phase and a first liquid phase; expanding the first liquidphase; and the second stream of heated carbon dioxide and the firstexpanded liquid phase undergoing a liquid/gas separation in a secondseparation flask in order to provide a second gaseous phase and a secondliquid phase.
 2. The process according to claim 1, further comprising:cooling at least one part of the natural gas in a third heat exchangerbefore the cryogenic process or before a reflux to the cryogenicprocess; dividing the second portion of the stream of recycled carbondioxide into a third portion and a fourth portion; expanding thenheating the third portion in the second heat exchanger, in order toprovide the second stream of heated carbon dioxide; cooling thenexpanding the fourth portion, then heating the expanded fourth portionin the third heat exchanger, in order to provide a third stream ofheated carbon dioxide; and recovering at least some of the hydrocarbonscontained in the third stream of heated carbon dioxide by liquid/gasseparation.
 3. The process according to claim 1, further comprisingoperating the first heat exchanger and the second heat exchanger atdifferent temperatures.
 4. The process according to claim 3, furthercomprising operating the first heat exchanger at a higher temperaturethan the second heat exchanger.
 5. The process according to claim 1,wherein the cryogenic process is a distillation.
 6. The processaccording to claim 1, wherein: the cooling of the second portion of thestream of recycled carbon dioxide is carried out in the second heatexchanger.
 7. The process according to claim 6, further comprisingcooling the stream of recycled carbon dioxide in the first heatexchanger before being divided into the first portion and the secondportion.
 8. The process according to claim 1, wherein the purifiednatural gas is heated, first in the second heat exchanger, then thefirst heat exchanger.
 9. The process according to claim 1, furthercomprising: expanding the second liquid phase; and a third stream ofheated carbon dioxide and the second expanded liquid phase undergoing aliquid/gas separation in a third separation flask in order to provide athird gaseous phase and a third liquid phase.
 10. The process accordingto claim 1, further comprising stabilizing the condensate of the secondliquid phase in order to provide a liquid phase rich in hydrocarbons anda gaseous phase rich in carbon dioxide.
 11. The process according toclaim 10, wherein the gaseous phase rich in carbon dioxide undergoes aliquid/gas separation in the second separation flask.
 12. The processaccording to claim 1, further comprising compressing and cooling thefirst gaseous phase and the second gaseous phase in order to provide anoutlet stream of carbon dioxide.
 13. The process according to one ofclaim 1 further comprising at least one of: mixing one part of thesecond liquid phase with the second portion of the stream of recycledcarbon dioxide; and mixing one part of an outlet stream of carbondioxide with the stream of recycled carbon dioxide.
 14. A process fortreating natural gas implemented in a plant, comprising: using acryogenic separation unit; using at least one line for natural gasconnected at the inlet of the cryogenic separation unit; using a linefor liquid carbon dioxide and a line for purified natural gasoriginating from the cryogenic separation unit; using a first heatexchanger passed through by at least one of the lines for natural gasconnected at the inlet of the cryogenic separation unit; using a secondheat exchanger passed through by at least one of the lines for naturalgas connected at the inlet of the cryogenic separation unit or by a linefor natural gas connected at the outlet of the cryogenic separation unitand feeding a reflux system; using a line for recycled carbon dioxideoriginating from the line for liquid carbon dioxide; using a line for afirst portion and a line for a second portion originating from the linefor recycled carbon dioxide, (a) the line for the first portion beingequipped with an expander and then passing through the first heatexchanger; (b) the line for the second portion being equipped with acooler; using a line for a third portion originating from the line forthe second portion, the line for the third portion being equipped withan expander and then passing through the second heat exchanger; andusing a gas/liquid separator fed by the line for the first portion andthe line for the third portion; separating the natural gas by acryogenic process in order to provide a stream of liquid carbon dioxidecontaining hydrocarbons and purified natural gas; cooling at least onepart of the natural gas; (a) in the first heat exchanger, (b) then inthe second heat exchanger before the cryogenic process or before thereflux to the cryogenic process; recovering at least one part of thestream of liquid carbon dioxide in order to provide a stream of recycledcarbon dioxide; dividing the stream of recycled carbon dioxide into thefirst portion and the second portion; expanding and then heating thefirst portion in the first heat exchanger, in order to provide a firststream of heated carbon dioxide; cooling the second portion, thenexpanding and then heating at least one part of the second portion inthe second heat exchanger, in order to provide a second stream of heatedcarbon dioxide; and recovering at least some of the hydrocarbonscontained in the first stream of heated carbon dioxide and in the secondstream of heated carbon dioxide by liquid/gas separation; the firststream of heated carbon dioxide undergoing a liquid/gas separation in afirst separation flask in order to provide a first gaseous phase and afirst liquid phase; expanding the first liquid phase; and the secondstream of heated carbon dioxide and the first expanded liquid phaseundergoing a liquid/gas separation in a second separation flask in orderto provide a second gaseous phase and a second liquid phase.